Portland cement
Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th century by Joseph Aspdin, and is usually made from limestone. It is a fine powder, produced by heating limestone and clay minerals in a kiln to form clinker, grinding the clinker, and adding 2 to 3 percent of gypsum. Several types of portland cement are available. The most common, called ordinary portland cement (OPC), is grey, but white portland cement is also available. Its name is derived from its resemblance to Portland stone which is quarried on the Isle of Portland in Dorset, England. It was named by Joseph Aspdin who obtained a patent for it in 1824. His son William Aspdin is regarded as the inventor of "modern" portland cement due to his developments in the 1840s.[1]
This article is about the building product of cement. For the Australian heritage-listed production site, see Portland Cement Works Precinct.The low cost and widespread availability of the limestone, shales, and other naturally occurring materials used in portland cement make it a relatively cheap building material. Its most common use is in the production of concrete, a composite material consisting of aggregate (gravel and sand), cement, and water.
Setting and hardening[edit]
Cement sets when mixed with water by way of a complex series of chemical reactions still only partly understood. The different constituents slowly crystallise, and the interlocking of their crystals gives cement its strength. Carbon dioxide is slowly absorbed to convert the portlandite (Ca(OH)2) into insoluble calcium carbonate. After the initial setting, immersion in warm water will speed up setting. Gypsum is added as an inhibitor to prevent flash (or quick) setting.
Safety issues[edit]
Bags of cement routinely have health and safety warnings printed on them, because not only is cement highly alkaline, but the setting process is also exothermic. As a result, wet cement is strongly caustic and can easily cause severe skin burns if not promptly washed off with water. Similarly, dry cement powder in contact with mucous membranes can cause severe eye or respiratory irritation.[19][20] The reaction of cement dust with moisture in the sinuses and lungs can also cause a chemical burn, as well as headaches, fatigue,[21] and lung cancer.[22]
The production of comparatively low-alkalinity cements (pH<11) is an area of ongoing investigation.[23]
In Scandinavia, France, and the United Kingdom, the level of chromium(VI), which is considered to be toxic and a major skin irritant, may not exceed 2 parts per million (ppm).
In the US, the Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for portland cement exposure in the workplace as 50 mppcf (million particles per cubic foot) over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 10 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday. At levels of 5000 mg/m3, portland cement is immediately dangerous to life and health.[24]
Due to the high temperatures inside cement kilns, combined with the oxidising (oxygen-rich) atmosphere and long residence times, cement kilns are used as a processing option for various types of waste streams; indeed, they efficiently destroy many hazardous organic compounds. The waste streams also often contain combustible materials which allow the substitution of part of the fossil fuel normally used in the process.
Waste materials used in cement kilns as a fuel supplement:[31]
Portland cement manufacture also has the potential to benefit from using industrial byproducts from the waste stream.[32] These include in particular: